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New Data on Gas Hydrates in U.S. Atlantic Margin

Gas hydrate occurs naturally in seafloor sediments when water and methane mix at moderate pressures and relatively low temperatures. It resembles ice and concentrates enormous amounts of methane in the global oceans and in permafrost areas. On the U.S. Atlantic Margin, BOEM (Bureau of Ocean Energy Management) discovered potential deep water gas hydrate deposits that guided the planning for the Mid-Atlantic Resource Imaging Experiment (MATRIX), carried out in 2018.

In the course of this experiment, scientists collected reflection seismic data along more than 2000 km of trackline. The MATRIX data captured images 1 to 3 km (about 3,300 to 9,800 ft) below the seafloor and gave new information about the distribution of a key seismic indicator linked to the presence of gas hydrates. The data also showed shallow gas deposits and structural features beneath some of the seafloor methane seeps discovered along the Atlantic Margin since 2011. The images included submarine landslide deposits, shallow faults, and sedimentary markers considered important in interpreting the history of the Atlantic Ocean basin to assess the risk of geologic hazards along the eastern seaboard. Four airguns were deployed to generate the acoustic energy for imaging the seafloor sediments. The seismic signals were recorded by 112 to 160 receivers towed behind the ship and arrayed within a streamer up to 1.2 kilometres (0.75 miles) long. The USGS also used 60 sonobuoys - expendable instruments that transmit seismic signals back to the ship. 

Research into the deposits of methane hydrate in the Atlantic Margin has been carried out for more than 10 years. In 2011, assessments of Marine Gas Hydrates and Associated Free Gas Distribution Offshore Uruguay performed the integration of a dense grid of different 2D seismic surveys acquired offshore data in order to identify the base of the gas hydrate stability zone and assess the gas hydrate and associated free gas distribution within the studied area. A probabilistic approach was considered in order to reflect the uncertainty of the interpretation, taking into account both the high and low side of the mapped area and reporting a final mean value.

Surveys conducted in 2014 discovered methane seeps on the upper continental slope, encompassing the shallowest depth range for gas hydrate stability (~505 to 575 m here) and the part of the slope updip from there. The seeps emitted methane at great depths so that it did not reach the atmosphere directly. Instead, the methane dissolved in the water column, where it appears to have remained for some time or was oxidized to carbon dioxide through microbial action. The scientists believed that such oxidation increased the ocean's acidity and could lead to local depletion of oxygen in the water column.

In 2017, scientists investigated the dynamics between gas hydrate stability and environmental changes from the height of the last glaciations to the present day. They used geophysical observations from offshore Svalbard to devise a coupled ice sheet/gas hydrate model and identified distinct phases of sub-glacial methane sequestration and subsequent release on ice sheet retreat that led to the formation of seafloor domes. Analyzing the evolution of these domes, they found that incursions of warm Atlantic bottom water initiated rapid gas hydrate dissociation and enhanced methane emissions across the continental margins of the Arctic Ocean.

Methane hydrate holds enormous potential for future energy production. It can release great amounts of energy which can be used for different purposes. Methane is a colourless and odourless gas which combusts completely to produce CO2, water and energy. There are many gas hydrate deposits in permafrost and deepwater marines. Methane can be produced from gas hydrates, which can be used as a sustainable energy source. Gas hydrates can also be used to store and transport natural gas. For gas hydrates to be stored effectively, a proper understanding of the processes involved in converting gas to hydrate and the processes required to prevent the hydrate from dissociating has to be gained. The storage of natural gas as hydrates will require the synthesis of the hydrate and its regasification. The advantage of this method is that it reduces the space requirements for the storage of natural gas. The stored gas can be used when natural gas demand is low and then be sold during periods of high demand. Recent research on hydrates has shown that only a minute amount of refrigeration, i.e. -7˚c is required to prevent dissociation of hydrate. The reason for the anomalous behaviour of hydrate is due to the presence of outer ice barriers that prevent inner-particle dissociation. The ice protective shell is a result of the re-freezing of water from the melted hydrate surface caused by an endothermic process.

The abundance of gas hydrate reserves can ultimately make them a sustainable energy resource all over the world. These hydrate reserves hold significant amounts of energy that is estimated to be more than twice the combined carbon of coal, conventional gas and petroleum reserves. The properties and formation of hydrates have to be studied and examined carefully because they are paramount to efficient and effective exploration and development of hydrate reserves.